We use ESR spectroscopy to study the behavior of manganese in molecular sieve type catalysts. As manganese occurs in these solids in highly dispersed states, several common techniques of inorganic chemistry (e.g. magnetic susceptibility or crystal structure determination) are of limited use. ESR, on the contrary, gives extremely detailed information on Mn coordination, especially when the values of the zero-field splitting parameters D and E can be evaluated. We have found that in A-type zeolites (with LTA topology), the Mn ion assumes three very different coordinations, which are easily distinguished through their D values of 100, 400 and 1500 G respectively. Especially in the latter case, when the D value gets of the same order of magnitude as the Zeeman energy of the X-band experiment, Q-band ESR analysis is indispensable for correct evaluation of the Mn coordination. We have also studied the intrazeolitic interaction of Mn with a cyclic triamine ligand, 1,4,7-triazacyclononane. In this case as well, we were able to define the coordination of the Mn through combined X and Q-band spectroscopies. We have recently discovered that these and related Mn molecular sieves are catalysts for the selective oxidation of alkanes and alkenes.